Field
Embodiments of the disclosure relate generally to ceramic materials and, in particular, to ceramics having improved corrosion resistance and impact damage.
Description of the Related Art
Gas turbine engines are a class of internal combustion engine commonly employed in power generation and aviation applications. In these engines, air enters the engine and is compressed to high pressure. The pressurized air is channeled through a combustion chamber, where a fuel is burned to produce heat. As a result, the temperature of the pressurized air is increased to an engine operating temperature, resulting in an increase in its velocity. This hot, high velocity, pressurized air is subsequently directed at a turbine, which extracts mechanical energy from the air by spinning. Depending upon the application, the spinning turbine may be employed to generate electrical power (e.g., gas-turbine generators) or to generate thrust/lift for aircraft (e.g., turbojet and turbofan engines).
In general, the thermal efficiency of gas turbine engine (the ratio of work output to heat input) is related to the difference between the temperature of the relatively cold input gas and the relatively hot, pressurized gas. That is, as the temperature difference between the intake air and the air at the engine operating temperature increases, so does the thermal efficiency of the engine (i.e., the more work is done for a given amount of input heat). Based upon this consideration, higher operating temperatures are favored, based purely on thermodynamic considerations.
In practical terms, though, the operating temperature of gas turbine engines, and therefore the thermal efficiency of the engine, is limited by the uppermost use temperature of materials forming the hot zone components of the gas-turbine engine (e.g., turbine blades, combustor liners, combustor shrouds, etc.). Traditionally, hot zone components have been formed from superalloys which possess high mechanical strength, creep resistance (resistance to time-dependent deformation under stress), and resistance to chemical attack (e.g., oxidation, corrosion, etc.), among other considerations. For example, modern superalloys can operate at temperatures up to approximately 1100° C.
To increase the temperature capability of superalloys in use, thermal barrier coatings may be applied to superalloy surfaces. For example, FIG. 1 presents a schematic illustration of a layered thermal barrier coating (TBC) system deposited upon a substrate such as the superalloy. The system can include a bond coat layer and a ceramic TBC layer (other layers may also be present but are omitted for simplicity). The bond coat can be applied to the substrate. In FIG. 1, the left surface of the substrate is assumed to be adjacent to a flow of cooling air and the right surface of the thermal barrier coating is assumed to be adjacent to a flow of hot gases. Accordingly, on the substrate side, the bond coat can protect the substrate against oxidation and corrosion. On the TBC side, the bond coat can provide adhesion to the TBC layer.
FIG. 1 further presents a schematic representation of temperature within the substrate, bond coat, and TBC during engine operation as a function of position (dashed line. For example, the TBC layer may thermally insulate the underlying superalloy from the operating temperature of the gas turbine engine (e.g., the hot gas temperature) and sustain a significant temperature difference between the load-bearing superalloy and the TBC surface. For example, air-cooled superalloy turbine blades including a protective TBC may be used at temperatures as high as about 200° C. above the melting temperature of the superalloy.
TBCs may undergo failure due to a number of different mechanisms. For example, foreign objects may enter the engine and impact on the TBC surface. When the foreign objects are relatively small (e.g., dust, etc.), such impacts may result in erosion of the TBC over time. Alternatively, when the foreign objects are larger (rocks, tools, etc.), such impacts may result in impact damage such as cracks, which can grow and lead to spallation of the TBC.